1. Field of the Invention
This invention relates in general to apparatus and methods for processing of semiconductor wafers in a diffusion furnace during a diffusion process and in particular relates to the method and apparatus for weighting a cantilever arm during processing of semiconductor wafers.
2. Description of Related Art
In processing semiconductor wafers in diffusion furnaces, the wafers have been inserted into a series of conventional quartz boats which are provided with slots for mounting the disc shaped wafers parallel to each other. The boats are then placed onto a cantilever arm which moves them into a diffusion furnace. So that the cantilever arm, boats and wafers clear the mouth and inner walls of the diffusion furnace and thus prevents damage to the semiconductor wafers, a suspension system is provided so as to support the cantilever arm for a standard deflection based on the weight of the boats and the wafers on the cantilever arm. Thus, the cantilever and suspension system are designed to support a fixed weight of boats and semiconductor wafers.
At times it is required to process less than the maximum number of semiconductor wafers and in the prior art it has been common to load boats in which the maximum number of semiconductor waters are not required with dummy wafers as a substitute so as to arrive at standard weight for the boats and wafers. For example, in the prior art, if six boats each capable of storing up to 50 wafers are utilized on the cantilever arm and less than 300 semiconductor wafers are to be manufactured then the empty spaces are filled with silicon dummy wafers. For example, if 200 semiconductor wafers are to be manufactured, 100 dummy wafers would be inserted to provide a load of 300 semiconductor wafers and the dummy wafers serve as weight substituted for products on the system.
In the prior art systems, it is desireable and necessary to inspect the quality of the semiconductor end product during a diffusion process and the load is monitored at several predetermined points within the series of boats. For example, in one particular arrangement, monitoring occurs at three points within the furnace and at selected points in three of the boats. The monitor wafers must be positioned at the preset points so that the monitor system can verify the quality of the manufacturing process across the entire product zone.
The prior art system of utilizing dummy wafers is expensive and a great deal of time is required to load and unload dummy wafers. The cost of dummy wafers also are an additional expense in the manufacturing process. The use of dummy wafers as weights requires additional storage space, cleaning, scribing, tracking and inspection which is unproductive. Also, dummy wafers can result in dopant cross-contamination and particulate generation.